Drop get-nix-kconfig.py (`nix-build get-nixos-kconfig.nix` does the job)

Refers to #77 and #63.

kconfig_hardened_check/config_files/distros/get-nix-kconfig.py

-#! /usr/bin/env nix-shell
-#! nix-shell -i python3 -p python3
-
-import json
-import os
-import shutil
-import subprocess
-import sys
-from tempfile import TemporaryDirectory
-
-
-def main() -> None:
-    proc = subprocess.run(
-        ["nix", "search", "-u", "--json", "^nixpkgs.linux_"], capture_output=True
-    )
-    data = json.loads(proc.stdout)
-    with TemporaryDirectory() as temp:
-        for pkg in data.keys():
-            symlink = os.path.join(temp, pkg)
-            res = subprocess.run(["nix", "build", f"{pkg}.configfile", "-o", symlink])
-            if res.returncode != 0:
-                print(f"failed to get configuration for {pkg}", file=sys.stderr)
-                continue
-            name = f"{pkg.replace('.', '-')}-config"
-            with open(name, "w") as dst, open(symlink) as src:
-                shutil.copyfileobj(src, dst)
-
-
-if __name__ == "__main__":
-    main()